Newton s Law of Motion

Transcription

1 chapter 5 Newton s Law of Motion Static system 1. Hanging two identical masses Context in the textbook: Section 5.3, combination of forces, Example 4. Vertical motion without friction 2. Elevator: Decelerating while rising 3. After the cable breaks... Context in the textbook: Complementary to Example 6 in Section 5.5. Motion along a horizontal plane without friction 4. Pushing two blocks Context in the textbook: Section 5.6, motion with constant force, Fig Motion along an incline 5. Pushing up an incline without friction Context in the textbook: Example 9 in Section 5.6. Motion in a system with an ideal pulley. 6. Hanging weight versus pulling force 7. Moving cart with pulley and blocks Context in the textbook: Examples 10 and 11 in Section PhysiQuiz Contents

2 α α M M Two identical masses are suspended symmetrically, as shown, so that T 1, the tension along string 1, is equal to T 3, the tension along string 3. Find T 2, the tension along string 2: A B C D T 2 T 1 cos T 1 sin 2T 1 cos 2T 1 sin Extra: Now lengthen string 2. This is done both to the right and to the left while maintaining the symmetry of the setup. Will T 2 increase, stay the same, or decrease? Explanation: String 2 is pulled to the left by the horizontal component of T 1 (that is, T 1x T 1 sin ) and to the right in equal magnitude by the horizontal component of T 3. Because tension is defined as the magnitude of either of the pulling forces, the tension along string 2 is T 1 sin. Answer B. Explanation extra: As string 2 is lengthened, decreases. Also notice from the vertical forces that T 1 cos Mg. Thus T 2 T 1 sin Mg tan decreases. 1. Hanging Two Identical Masses PhysiQuiz 39

3 v a A woman in an elevator is standing on a scale. When the elevator is at rest, the scale reading is S 50 kg, corresponding to a weight of approximately 500 N. While the elevator is moving upward, it is decelerating with a g/10. The new scale reading will be which of the following? A B C S (in kg) Hint: Newton s Second Law is mg F scale ma. Extra: Determine F scale in N. Explanation: Intuitively, as an ascending elevator comes to stop, a person feels lighter that is, less than 50 kg. This is confirmed by Newton s Second Law, which tells us that F scale mg ma mg. Answer A. Explanation extra: Let the downward direction of a be positive. The force equation takes the form mg F scale mg/10. F scale mg mg/ (1 1/10) 450 N. So the scale reading is about 450/10 45 kg. 40 PhysiQuiz 2. Elevator: Decelerating While Rising

4 v A woman in an elevator is standing on a scale. As the elevator ascends at a constant speed, the scale reading is S 50 kg. In force units it is approximately 500 N. If the cable suddenly breaks, the new scale reading will be which of the following? A B C D S (in kg) S 0 0 S 50 S 50 S 50 Hint: Use mg F scale ma. Extra: Soon after that, the elevator starts descending. Will the scale reading change? If so, describe the change. Explanation: Using the equation given in the hint, with a g, we find that F scale 0, or S 0. Answer A. Explanation extra: After the cable breaks, a g whether the elevator is rising or falling. This implies that after the break, S 0, regardless whether the elevator is rising or falling. 3. After the Cable Breaks... PhysiQuiz 41

5 F Block 2 Block 1 m 2 m 1 In this figure, let F 21 be the force exerted on block 2 from block 1. If the system accelerates with a constant acceleration a, which of the following is the equation of motion from Newton s Second Law for block 2? A B C F m 2 a F 21 m 2 a F F 21 m 2 a Extra: What is F F 21 in terms of m 1, m 2, and a? What happens to the motion of block 1 when F F 21? Explanation: The force on block 2 is F 21. Answer B. Explanation extra: Applying Newton s Second Law to block 1, F F 21 m 1 a. Notice that this difference is proportional to the acceleration of block 1. When F F 21, a 0. This occurs when block 1 is not moving or if block 1 is moving with a constant speed. 42 PhysiQuiz 4. Pushing Two Blocks

6 F θ M θ A block is pushed up a frictionless inclined plane as shown. Determine the magnitude of the horizontal force vector, F F, that will keep the block sliding up the incline with a constant velocity: A B C D F mg sin mg cos mg tan mg cot Extra: Is the force required for the block to stay at rest greater than, equal to, or less than F given here? Explanation: To push the block up the inclined plane at a constant velocity, there must be no net force on the block along the direction of the incline. Therefore the component of F along the incline must cancel the component of the weight along the incline. The reader should sketch a vector diagram to show that this cancellation implies that F cos mg sin, or F mg tan. Answer C. Explanation extra: Because both the stationary and constant velocity cases have a 0, the cancellation condition remains valid. Therefore the force is the same for both cases. 5. Pushing up an Incline without Friction PhysiQuiz 43

7 A m A m m F = mg I II Consider the two possible accelerations of block A across the table toward the pulley. If friction is assumed to be negligible in each case, then the acceleration of block A is which of the following? A Greater in case I. B Greater in case II. C Both are the same. Extra: Find the ratio of a I /a II. Explanation: Applying Newton s Second Law to case I gives mg (m m)a I. Or a I g/2. The corresponding equation for case II gives mg ma II. It gives a II g. So the second case has a greater acceleration. Answer B. Explanation extra: From the explanation section, it follows that the ratio a I /a II 1/2. 44 PhysiQuiz 6. Hanging Weight versus Pulling Force

8 Block 1 F M Block 2 In this setup consider the surfaces in contact with the blocks, the wheels of the cart, and the pulley all to be frictionless. Let m 1 3m 2. The force F is adjusted so that block 1 is stationary with respect to the horizontal surface of the cart. The acceleration of the cart is a. The tension of the string is given by which of the following? A B C Tension of the string m 1 a (m 1 m 2 )a (m 1 m 2 )g Extra: Determine a. Explanation: Applying Newton s Second Law to block 1 gives T 1 m 1 a. Answer A. Explanation extra: Because the string tension is also given by m 2 g, T 1 m 1 a m 2 g. Solving for a gives a m 2 g/m 1 g/3. 7. Moving Cart with Pulley and Blocks PhysiQuiz 45

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